All accessory proteins of HIV-1, the ethiologic agent of AIDS, are thought to optimize viral replication and propagation in vivo. Among them, Vpu antagonizes Tetherin, a cellular factor that inhibits viral particle release. Downregulation of cell-surface Tetherin by Vpu is believed to prevent incorporation of this restriction factor into nascent viral particles, which would impede the formation of a Tetherin-derived protein anchor that bridges the virus to the plasma membrane of the infected cell. This thesis presents our studies on cellular mechanisms governing Tetherin antagonism by Vpu. A directed mutagenesis approach first identified two regions encompassing determinants of the localization of Vpu in the trans-Golgi network, and it correlated this intracellular distribution with enhanced release viral particle. Pulse-chase experiments in cellular systems wherein Tetherin was endogenously expressed showed that Vpu-induced Tetherin degradation is dispensable for restriction. In contrast, both a flow cytometry-based Tetherin re-expression assay and confocal microscopy analyses demonstrated that Vpu-mediated sequestration of Tetherin in the trans-Golgi network, a phenomenon that appeared to be triggered by the transmembrane association of the two proteins, was necessary for release inhibition. Vpu inducible expression in flow cytometry-based experiments provided evidence for an optimal antagonism of Tetherin at 6h after Vpu expression, following the interruption of Tetherin re-supply and a to the modest acceleration of the natural clearance of surface-localized Tetherin. Our work supports a model in which Tetherin sequestration in the trans-Golgi network prevents its re-supply, which, combined with its clearance from the surface, imposes a new equilibrium at the plasma membrane that is incompatible with the restriction of viral particle release. Overall, this thesis sheds light on the processes by which Vpu enhances the secretion of mature viruses and it establishes a mechanistic basis that could serve as starting point for the development of strategies aimed at interfering with Tetherin functions.